My 4 month old baby has an allergic reaction to milk formula and has switched to soy without problems. when should he have an allergy test?

No rush if soy OK: Was the reaction skin, respiratory or gastrointestinal (gi)? Was there blood in the feces? If symptoms were completely GI & associated with pallor or floppiness think enterocolitis. This non-ige mediated milk allergy reaction may persist for several years. Only valid test is oral challenge carefully performed by an experienced allergist. For skin & respiratory symptoms skin or blood test at 1 year.

12 months: Most milk allergies are outgrown-often by the age of 2. Blood tests can be done annually to measure specific allergy antibody to milk and when the level has dropped significantly an in-office oral challenge could be arranged. Would recommend the first blood test after 12 months of age.